The use of catalytic combustion chambers in gas turbine engines is a desirable aim, because of the benefits in the reductions of combustion chamber emissions, particularly nitrogen oxides (NOx). The reduction in NOx is due to the lower operating temperatures and the use of much weaker fuel and air ratios than conventional combustion chambers.
In catalytic combustion chambers it is known to use ceramic, or metallic, honeycomb monoliths which are coated with a suitable catalyst. It is also known to use honeycomb monoliths which contain a suitable catalyst or are formed from a suitable catalyst.
It is also known to arrange several of the honeycomb monoliths in flow series such that there is a progressive reduction in the cross-sectional area of the cells of the honeycomb from one honeycomb monolith to an adjacent honeycomb monolith, in the direction of flow. The honeycomb cell size may vary and the cross-gectional area for flow may vary. The smaller honeycomb cell size has the effect of providing a high geometric surface area per unit volume, which may increase the available catalyst area per unit volume, which in turn may increase the catalytic reaction rate per unit volume and hence reduce emissions of unburned hydrocarbons.
In catalytic combustion chambers there is an optimum temperature range at which catalytic reaction on the catalyst will occur. At temperatures below the optimum temperature range the rate of catalytic reaction will be very low, whilst at temperatures above the optimum temperature range the catalytic reaction diminishes due to damage to the catalyst, for example because of sintering, or phase transition e.g. palladium oxide changes to palladium, and lose its activity. However the catalytic activity of the catalyst is never likely to be zero. Different catalysts have different optimum temperature ranges. Thus some catalysts have good lower temperature capabilities, i.e. will operate at relatively low temperatures around 350.degree. C. to 400.degree. C., but have poor higher temperature capabilities. Other catalysts have good higher temperature capabilities, but poor lower temperature capabilities. Also a gas turbine engine operates over a wide operating range. Currently there is no known catalyst which has an acceptable level of activity across the entire operating temperature range of a gas turbine engine combustion chamber. This makes it necessary to have a series of catalyst coated honeycomb monoliths arranged in series in a combustion chamber, with catalysts having good lower temperature capabilities on the first honeycomb monolith and catalysts having progressively increasing higher temperature capabilities such that the catalyst on the last honeycomb monolith has the best higher temperature capability. Thus there may be two or more catalyst coated honeycomb monoliths arranged in flow series in a catalytic combustion chamber. Usually it is arranged that the temperature downstream of the last catalyst coated honeycomb monolith is sufficient to support homogeneous gas phase reactions.
In catalytic combustion chambers hydrocarbon fuel and air are mixed and supplied to the catalyst coated honeycomb monoliths, or honeycomb monoliths formed from, or containing catalyst. The hydrocarbon fuel and air mixture diffuses to the catalyst coated surfaces of the honeycomb monoliths and reacts on the active sites, at and within the surface.
In one known catalytic combustion chamber a pilot combustor, or pre-burner, is provided to burn some of the fuel to preheat the first catalytic combustion zone to thee optimum temperature range. A main fuel injector positioned upstream of the first catalytic combustion zone, is provided to supply fuel to the first catalytic combustion zone. The second and subsequent catalytic combustion zones receive unburned fuel from the first catalytic combustion zone.
It has been proposed to provide a catalytic combustion chamber with a pilot combustor, or pre-burner, to burn some of the fuel to preheat the first catalytic combustion zone to the optimum temperature range. A main fuel injector, positioned upstream of the first catalytic combustion zone, is provided to supply fuel to the first catalytic combustion zone. An additional fuel injector, positioned between the first and second catalytic combustion zones, is provided to supply additional fuel to the second catalytic combustion zone.
A problem associated with catalytic combustion chambers is that there is a possibility that one or more of the catalytic combustion zones, may become overheated leading to deactivation of the catalyst. It is also necessary to ensure that the temperature downstream of the last catalytic combustion zone is sufficiently high to maintain homogeneous gas phase reactions.